The present invention relates to a method and a device for monitoring the function of a valve stroke adjusting device of an internal combustion engine during cold starts.
Modern internal combustion engines are equipped with serial camshaft adjusting systems for improving power and torque. All such systems function according to the principle of rotation of the phasing of the camshaft relative to the crankshaft by means of hydraulic or hydromechanical adjustment elements.
However, such systems are scarcely able to meet rising demands on exhaust quality, fuel efficiency, power and torque. There are already developments on the market such as the multistep or stepless valve stroke adjusting system.
German patent publication DE 100 16 103 A1 describes a valve stroke adjusting device. The valve stroke is adjusted so that the valve blocks the gas flow of the air/fuel mixture to a greater or lesser degree, which decisively influences how the combustion chambers are filled with and emptied of the combustion gas.
Besides the pure adjusting function, importance is also placed on aspects such as a simple, robust construction, small friction losses, reliable function over the entire useful life, applicability to existing cylinder head constructions, and so on. It should be possible to use the device in both Otto and diesel engines on the intake and/or exhaust sides.
The proper functioning of a valve stroke adjusting device is an important factor in reducing the fuel consumption and exhaust emission of an internal combustion engine. Therefore the legislature has mandated that whenever an internal combustion engine is equipped with a valve stroke adjustment system, the functionality of such system must be monitored.
One type of valve stroke adjusting system, for example, switches from a small valve stroke to a large stroke immediately upon the ignition of the engine during a prescribed cold start phase. Known monitoring systems have problems during this cold start phase or have complicated and expensive constructions.
European patent publication EP 1 467 068 A1 describes a method in which the valve stroke adjustment is evaluated by a lambda probe with reference to the exhaust gas composition. This known system has exhibited the drawback that the lambda probes are not operational during cold starts.
Another possibility for monitoring functionality is to detect uneven running in individual cylinders and compare it with predetermined threshold values. However, there is no notable unevenness during cold starts.
Still another possibility for monitoring functionality is to evaluate the signal of the airflow meter. The drawback of this method is that the signal qualities of the airflow meter are heavily dependent on other factors.
A solution which involves a large expenditure for sensor technology is to measure the pressures in front of the intake valves. Different pressure levels can be correlated with different valve stroke profiles. These pressure levels are compared with values stored in engine operating maps.
Japanese patent publication JP 61 294 03 discloses a valve stroke monitoring device having a solenoid valve for opening and closing an air intake line in an operation section. The solenoid valve is closed for a definite time-span by means of a diagnostic start signal, and when the valve opens, an abnormality signal is emitted if a valve opening value is below a predetermined threshold value. A stroke detector measures the valve's opening level. It is also expensive to apply this known solution to monitoring the functionality of a valve stroke adjusting device.
An object of the present invention is to provide an improved method and a relatively inexpensive device for monitoring the functionality of a valve stroke adjusting device of an internal combustion engine during cold starts, which device is optimally reliable.
The method of the invention claimed and the corresponding device for monitoring the functionality of a valve stroke adjusting device of an internal combustion engine during cold starts exhibit the advantage that monitoring can be carried out inexpensively and precisely.
The central idea of the invention is that the monitoring of the functionality of a valve stroke adjusting device of an internal combustion engine during cold starts is based on the integral component of the throttle control. The advantages of this type of monitoring are that it does not require any extra expenditure for sensors, and monitoring can be carried out a few seconds after the start-up of the engine.
According to a preferred embodiment, there is a measuring step to determine the time elapsed since the start of the cold start phase and whether said time-span is at least equal to a predetermined time-span, which condition must be fulfilled before the acquisition of the integral component in the cold start phase will be performed. The advantage of this is that it prevents faulty detection at the beginning of the cold start phase.
According to a further preferred development, the threshold value to be compared with the acquired integral component is chosen from a number of predetermined threshold values on the basis of the measured time-span. This way, a time dependency of the integral components is taken into account.
According to a further preferred embodiment, the internal combustion engine comprises two cylinder banks with separate valve stroke adjustment by the valve stroke adjusting device, and the comparison and evaluation steps proceed as follows: a first comparison of the integral component acquired in the cold start phase with a predetermined first threshold value; first evaluation of the functionality of the valve stroke adjusting device of both cylinder banks based on the result of the first comparison; if the first evaluation indicates that the functionality of the valve stroke adjusting device of at least one of the two cylinder banks is faulty, a second comparison of the integral component acquired in the cold start phase with a predetermined second threshold value; a second evaluation of the functionality of the valve stroke adjusting device of the two cylinder banks based on the second result of the second comparison. This allows for incremental diagnosis.
According to a further preferred embodiment, the threshold value or values are stored in a storage device.
According to a further preferred embodiment, the threshold value or values are adapted before the comparison on the basis of a throttle setpoint value adaptation. This way, aging processes and error tolerances can be taken into account.
According to a further preferred embodiment, errors detected in the functioning of the valve stroke adjusting device are indicated by a signaling means. This way, the driver can be informed of errors.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
Identical or functionally equivalent elements are assigned the same reference signs.